1. Field of the Invention
The present invention relates to an electrophotographic copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to a belt type fixing device for use in an electrophotographic image forming apparatus.
2. Description of the Background Art
It is a common practice with an electrophotographic image forming apparatus to use a fixing device including a heat roller or a heat belt. A problem with such a fixing device is that a sheet, jamming the outlet of the fixing device and carrying a solid image with a high toner ratio thereon, is apt to adhere to the heat roller or the heat belt because toner melted by heat is highly adhesive, as described in, e.g., Japanese patent laid-open publication No. 2004-258581. If a sheet is absent in the sensing range of a sheet sensor with the above sheet adhering to the heat roller or the heat belt, it is likely that a jam is determined to have already been settled, so that the heat roller or the heat belt is continuously heated.
Japanese patent laid-open publication No. 2001-20197, for example, teaches that the belt type fixing device is advantageous over the roller type of fixing device in that when the belt is passed over a plurality of shafts, the sheet, adhering to the belt, can be more easily removed from the belt than from the roller due to the angle of the belt. However, in the case of an oilless color fixing system, in particular, the sheet does not smoothly part from the belt, but is apt to remain on the belt, because a great amount of toner is deposited on the sheet and because oil is absent.
Assume that a conventional contact type temperature sensor is positioned in the sheet passing range of a fixing device of the type using a belt. Then, if the surface layer of the belt is covered with fluorine, the temperature sensor is apt to scratch the surface layer. To solve this problem, a non-contact type temperature sensor may be used, as taught in, e.g., Japanese patent laid-open publication Nos. 11-305590 and 2000-242134 by way of example. A non-contact type temperature sensor, however, brings about another problem that if a sheet jams a gap between the belt and the temperature sensor, then the temperature sensor senses the temperature of the sheet as the temperature of the belt. As a result, it is likely that a heat source disposed in a heat roller is turned on to heat the belt to an unexpected level and melts the surface layer of the belt or otherwise damages the belt.
As stated above, if a sheet adheres to the belt due to some trouble occurred during conveyance and if a sheet is absent in the sensing range of the sheet sensor, then a sheet jam is determined to have been settled with the result that the heat source starts heating the belt with the sheet remaining on the belt. Further, if a sheet remains in the gap between the non-contact type sheet sensor and the belt, then the sheet sensor senses the temperature of the sheet as the temperature of the belt and causes the heat source of the heat roller to generate heat.